A leased line is a permanent (permanent, so-called "dedicated") connection between two communication partners via a circuit-switched telecommunication network. In contrast to this, with a dial-up line, the connection is temporarily established and cleared by means of a dialing process.
All types of data can be transmitted via the connection, for example analog (e.g. telephone conversation ) or digital (e.g. direct data connection). The connection itself can be carried out physically, e.g. B. as a copper wire pair from terminal to terminal, or virtually, as part of a higher-level infrastructure such as SDH or ATM - Virtual Circuit . The connection of a permanent data line is bit transparent .
The connection cannot be established by the subscriber using a dialing method, such as multi-frequency dialing , but must be established by the network operator. Leased lines therefore have no phone number .
The term leased line is sometimes used to designate a long-term connection of any type and quality. The simplest form of such a dedicated line (not switched by the provider) is a permanently established dial-up line via the telephone network. Here the modems of the participants are programmed so that they dial one another.
In technical jargon , dedicated lines are referred to as direct data connection (DDV) or standard leased line (SFV) according to the product names of Deutsche Telekom . Deutsche Telekom's standard analog leased lines could be used until September 30, 2009. Deutsche Telekom coined the expression direct call network for telephone lines, a dedicated line was referred to as a main connection for direct calls (HfD). Further designations are: leased line connection (MLV) or leased line.
Originally, a dedicated line was actually implemented as a wire connection that was switched between the two end points. In the meantime, however, intelligent multiplex systems are used for this , which can be flexibly switched and managed by the network operator from a central network management system.
Most of the leased lines installed in Germany on copper lines have data transmission rates of up to 2 Mbit / s. The double copper wires of the fixed network are usually sufficient for these connections . In the past, coaxial cables were often used for higher bandwidths ; these are now increasingly being replaced by optical fibers .
Leased lines on fiber optic cables are only offered with bandwidth restrictions by some network operators. For example, For example, Deutsche Telekom offers a dedicated line with data transfer rates of up to 100 Mbit / s that can be scaled in steps of 10 Mbit / s under the name DDV-M Ethernet100 .
Since around 2004, alternative network operators have been increasingly entering the German market that offer fiber optic cables (or a WDM band of a fiber) without bandwidth or usage restrictions. This connection is known as dark fiber because the customer himself has to provide the "light" on the line.
With political support among other things, the nationwide fiber optic expansion has been driven by all relevant network operators since 2017. This expansion is accelerated by means of the new micro trenching process, as the trenches required for laying fiber optic cables only have to be 15 to 20 cm wide. Large openings in sidewalks are therefore no longer necessary.
Data transfer rate
- For the internationally possible speeds see: Plesiochronous digital hierarchy
Leased lines can be rented with data transmission rates of 2 Mbit / s up to 100 Gbit / s. (As of 2018)
The most widespread types were initially E1 (up to 2 Mbit / s), E3 (up to 34 Mbit / s) and STM1 (up to 155 Mbit / s). With the gradual replacement of SDH, ethernet-based connections with line bandwidths of 10 Mbit / s, 100 Mbit / s, 1,000 Mbit / s and 10,000 Mbit / s, on which smaller sub-bit rates can then also be configured as required, are becoming increasingly popular .
As a rule, the dedicated line is rented. The price depends, among other things, on the length of the line, the gross data rate provided and the line medium. In addition, there may be costs for data volume with bundled offers .
The technical properties of the dedicated lines, especially the quality requirements such as reliability and bit error frequency , are defined in Germany by the responsible federal authority, the Federal Network Agency (formerly the regulatory authority for telecommunications and post ). If dedicated lines are implemented via fiber optic connections, one can assume a particularly low error rate in the physical layer . Important dedicated lines can be additionally protected by preparing alternative channels (see Protection ). 98.5% annual availability is typical for copper cables, 99.5% for fiber optics. However, these availabilities result primarily from the recovery times in the event of a complete failure of a connection and less from sporadic bit errors.
Different interfaces are used for dedicated lines. The data transmission device for connection to a 2 Mbit / s dedicated line often has an X.21 or G.703 interface. These interfaces are then used by the respective routers of the connected networks at the two end points of a dedicated line .
Leased line to the Internet
A dedicated line is often used to connect to the Internet , especially by companies. The leased line then leads to the Point of Presence (PoP) of an Internet service provider . If a network is connected to the Internet in this way via a dedicated line, this usually also includes the assignment of at least one fixed, public IP address . These leased lines are particularly suitable for operating servers on the Internet. Often leased lines are also used to connect two private (sub) networks; no public IP addresses are assigned here. In the case of leased lines of the second type, only the provision by the provider is charged, while in the case of leased lines to the Internet, in addition to the provision, the transferred data volume must also be paid for.
Permanent DSL connection
The connection via a DSL modem to the Internet is sometimes referred to as a leased line, for example real (S) DSL -based leased lines, such as those used by B. Arcor offers or QSC offered until April 2, 2009 for private customers. Quasi-permanent lines with regular forced disconnection and changing IP addresses, such as those used in B. Deutsche Telekom or Telekom Austria offer, are not leased lines.